In the copolymerization process in which the monomers carbon monoxide and ethylenically unsaturated compounds are reacted under polymerization conditions in the presence of a catalyst system, based on a palladium compound, a bidentate bisphosphine ligand and a source of anions, linear substantially alternating copolymers of carbon monoxide and said ethylenically unsaturated compound(s) are produced. These copolymers have established utility in various outlets for thermoplastics. They may be further processed by conventional techniques into films, sheets, plates, fibres and shaped articles such as containers for food and drinks and various parts in the automotive industry. A suitable method for the preparation of the linear copolymers, which is usually performed in batch operation, is described in EP-181014.
The batchwise preparation of the copolymers is generally carried out by introducing a solution of the catalyst into a reactor which contains the monomers and a diluent in which the copolymers are insoluble or virtually insoluble. Various liquids may be considered for use as diluent in the copolymerization reaction. Lower aliphatic alcohols are preferred. Methanol is particularly preferred since it has been found that the catalysts display their highest level of activity if a diluent of this type is used in the copolymerization reaction.
Accordingly, it would be advantageous to introduce a catalyst solution into the reactor along with a lower aliphatic alcohol as solvent. However, it has been observed that the selection of a lower aliphatic alcohol as solvent for the catalyst system entails two problems. One problem relates to the preparation of the catalyst and another involves its storability. Initially, catalyst systems were applied which comprised a palladium compound and an anion source, and a bidentate bisphosphine ligand of the formula (Q).sub.2 P-R-P(Q).sub.2 whereby Q represents an aromatic hydrocarbyl group and R represents a bivalent organic bridging group containing at least two carbon atoms in the bridge. In a subsequent investigation it was found that the performance of the catalyst system could be improved significantly by replacing the ligand of the formula (Q).sub.2 P-R-P(Q).sub.2 by a ligand of formula R.sup.1 R.sup.2 P-R-PR.sup.3 R.sup.4 wherein R.sup.1, R.sup.2, R.sup.3 and R.sup.4 independently represent alkoxy substituted aromatic hydrocarbyl groups. However, the solubility of these substituted bisphosphine ligands in methanol and other lower aliphatic alcohols proved to be poor and hence it was proposed to perform the preparation of the catalyst in another solvent such as toluene.
Another problem encountered with the use of a lower aliphatic alcohol, in particular methanol, as catalyst solvent, consists in the short shelflife of the catalysts thus prepared. This is apparent from the significant plating-out of palladium present in the catalyst system. In principle, this problem can be circumvented by preparing a fresh catalyst solution prior to each batch operation of the copolymerization process. Obviously it would be desirable to prepare a larger quantity of catalyst which can be stored so that a portion of the stored catalyst can be applied to each batch as needed.
Problems encountered with the use of methanol as solvent in the preparation of the catalyst system are discussed in EP-360359. According to this document, the storability of the catalyst is improved by using an aprotic polar solvent in the preparation of the catalyst. Acetone is particularly preferred.
It appears that the solubility of the ligands of the formula R.sup.1 R.sup.2 P-R-PR.sup.3 R.sup.4 is higher in acetone than in methanol. In multi-batch operations of the copolymerization process, it is common practice to separate the product by removing the diluent from the reaction medium, (e.g. by distillation) and to recycle the diluent to the next batch. Since fresh catalyst is supplied for each batch, this implies--following the procedure of EP-360359--that the amount of acetone builds up after each recycle. This necessitates a purifying treatment of the recycle stream, or a significant bleed of the stream. Both measures are unattractive for economic reasons.
Preparation of the catalyst as a solution in acetone and use of the same liquid as diluent in the co-polymerization reaction might also be considered. However, as previously noted, the use of acetone as the only diluent in the copolymerization reaction is less attractive in view of the lower level of activity of the catalysts in that environment.
Surprisingly, it has now been found that by first combining the palladium compound, the bisphosphine ligand and a lower aliphatic alcohol as solvent and then admixing the resulting combination with an acid after a generation period related to the temperature at which the catalyst preparation is performed, a catalyst solution of good storability is obtained. This allows the use of a single liquid in the preparation of the catalyst and in the production of the copolymers.